iscsi_ibft: Fix finding Broadcom specific ibft sign

Search for Broadcom specific ibft sign "BIFT"
along with other possible values on UEFI

This patch is fix for regression introduced in
“935a9fee”.
https://lkml.org/lkml/2011/12/16/353



This impacts Broadcom CNA for iSCSI Boot on UEFI platform.

Signed-off-by: Vikas Chaudhary <vikas.chaudhary@qlogic.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Mike Christie <michaelc@cs.wisc.edu>